1. Field of the Invention
The present invention relates to a cell comprising a plurality of generally elliptic cylindrical wound generating elements, such as a nonaqueous electrolytic secondary cell.
2. Description of the Related Art
In conventional prismatic cells of large discharge capacity, e.g. a nonaqueous electrolytic secondary cell, a plurality of elliptic cylindrical wound power-generating elements are superposed and housed into a cell case. In the nonaqueous electrolytic secondary cell, as shown in FIG. 9, four elliptic cylindrical wound power-generating elements 3 have respective peripheral flat side faces superposed on one another and are housed into a cell case 1. FIG. 10 illustrates a connecting structure of the power-generating elements 3 and current collectors 4. The illustrated nonaqueous electrolytic secondary cell comprises four elliptic cylindrical wound power-generating elements 3 which are connected in parallel to one another. Each power-generating element 3 includes a strip-like positive electrode and a strip-like negative electrode which are both wound into the shape of an elliptic cylindrical winding with a separator interposed therebetween. The positive electrode is made of a strip-like aluminum foil having a surface on which a positive active material is carried, whereas the negative electrode is made of a strip-like copper foil having a surface on which a negative active material is carried.
The active material is applied to a substrate of each electrode except for at least one of the edges thereof. The power-generating element 3 is wound so that a portion of the positive electrode to which the active material is applied is substantially opposed to a portion of the negative electrode to which the active material is applied with the separator 3c being interposed therebetween. The active material is not applied to a substrate edge 3a′ of the positive electrode, and the active material is not applied to a substrate edge 3b′ of the negative electrode. The substrate edges 3a′ and 3b′ are disposed so as to be opposed to each other with the opposed active-material-applied portions being interposed therebetween.
The active-material-applied face of the negative electrode 3b is disposed on an outer peripheral side face of the wound power-generating element 3, or another substrate of the positive or negative electrode 3a or 3b is wound on the peripheral side face of the wound power-generating element 3 once, or only another separator is wound on the peripheral side face of the wound power-generating element 3. A tape made from polypropylene resin is applied to an end of the wound power-generating element 3 so that the power-generating element is maintained in a wound state.
In the power-generating element 3 of the elliptic cylindrical wound type, each of the substrate edges 3a′ and 3b′ comprises arc portions that are curved and overlapped, and a flat overlapped straight portion. Two current collectors 4 are connected to the straight portions of the substrate edges 3a′ and 3b′, respectively. Each current collector 4 includes recesses of corrugated electrode-connecting portions 4b. The substrate edges 3a′ and 3b′ are put into the recesses to be welded as shown in FIG. 9, thereby serving as leads connecting the positive and negative plates to the positive and negative terminals, respectively. An aluminum alloy plate is usually used as the current collector 4 connected to the substrate edge 3a′ of the positive electrode, whereas a copper alloy plate is usually used as the current collector 4 connected to the substrate edge 3b′ of the negative electrode.
The power-generating element 3 including two current collectors 4 connected as described above is inserted into a casing-like stainless-steel cell case 1 from above and an upper open end of the case 1 is closed by a cover plate (not shown), as shown in FIG. 9. A periphery of the cover plate is welded to the case 1 thereby to seal the case. A predetermined amount of electrolyte is poured through a filler hole into the interior of the case 1. The filler hole is thereafter closed, whereupon a nonaqueous electrolytic secondary cell is thus provided. The positive and negative terminals 4b and 5b of the respective collector tabs 4 and 5 extend through the cover plate while being insulated from the cover plate.
In the cell comprising the elliptic cylindrical wound power-generating elements 3, however, the peripheral flat portion of each power-generating element 3 is gradually curved to be expanded with repeated charge and discharge, whereupon each power-generating element 3 tends to change from the elliptic cylindrical shape to a cylindrical shape. When the power-generating element 3 is thus deformed, a distance between the wound positive and negative electrodes is increased such that the performance of the cell is reduced. Furthermore, the expansion of the peripheral flat portion of the power-generating element 3 also forces the cell case 1 to expand.
Furthermore, the active material applied to an outermost negative electrode may come into contact with an inner surface of the cell case 1 to fall off when each power-generating element 3 is put into the case. Additionally, even where the separator is wound around an outermost peripheral face of the power-generating element 3, the separator may be damaged when the power-generating element 3 is housed into the case, whereupon an active material applied to inner electrodes may fall off.
Furthermore, pieces of the active material that have fallen off move in the cell case 1 during use of the cell, resulting in a tiny short circuit between the positive and negative electrodes. The discharge capacity of the cell reduces rapidly since discharge of a feeble current continuously takes place at a short-circuit portion upon occurrence of such a tiny short circuit. This poses a problem.
The substrate edges 3a′ and 3b′ of the arc portion of the power-generating element 3 come into contact with the inner face of the cell case 1 to be bent when the elliptic cylindrical wound power-generating element 3 is accommodated in the cell case 1. In this case, the current collector is sometimes broken and falls off. Further, when an impact force is applied to the substrate edges 3a′ and 3b′, the active material sometimes falls off from the electrode part that is located in the vicinity of the substrate edges 3a′ and 3b′. In this case, too, broken pieces of the substrate and fallen pieces of the active substance may adhere to the positive or negative electrode, resulting in a tiny short circuit.
When the power-generating element 3 is manufactured, as typically shown in FIG. 10, the separators 3c are wound on a core 9 by a half turn or one turn while being pressed by a pair of rollers 10, and thereafter, the positive and negative electrodes 3a and 3b are wound onto the respective separators 3c together with the separators 3c. Thus, the positive and negative electrodes 3a and 3b are overlapped with the separators 3c being interposed therebetween thereby to be wound on the core 9 having an elliptic cylindrical section. The core 9 is made of a resin having high electrolyte resistance, e.g. polyethylene terephthalate (PET).
A relatively large clamping force is applied to an arc portion S (see FIG. 7) of the power-generating element 3 manufactured as described above such that the positive and negative electrodes 3a and 3b and separators 3c are dense. On the contrary, the positive and negative electrodes 3a and 3b and separators 3c are sparse in a straight portion Q. Accordingly, the electrolyte is hard to permeate into the arc portion S. Particularly when the separator is excessively pressed such that porosity thereof is reduced, each separator 3c cannot reserve a sufficient amount of electrolyte. This adversely affects the charge and discharge characteristics of the cell, resulting in variations in the characteristics between the power-generating elements 3 and between the cells.
On the other hand, when the clamping force is excessively small, an interelectrode distance becomes unstable, whereupon the charge and discharge characteristics vary among power-generating elements and among cells. Accordingly, in the cell including the elliptic cylindrical wound generating elements, it is important that the interelectrode distance between the positive and negative electrodes should not become unstable in the straight portion where the electrodes and separators are wound by a relatively small force, and that the electrolyte should be maintained in the separators sufficiently and homogeneously.
Reference symbol d designates a major axis of a cross-sectional ellipse in the power-generating element 3, and reference symbol w designates a minor axis thickness of a cross-sectional ellipse in the power-generating element 3. When the value of (d−w) is large, the distance between the positive and negative electrodes is increased in a central flat portion of the peripheral face of the power-generating element 3 even if the peripheral face of the power-generating element 3 is fastened tight by a sheet member. Accordingly, the value of (d−w) is required to be within a proper range.